Differential Detection of Multilevel Continuous Phase Modulation for Radio Applications

摘要

Continuous phase modulation is a good choice, especially for radio applications, because of the combination of its constant envelope property and good bandwidth efficiency. In radio relay systems operating at very high radio frequencies (RF) and also in the field of mobile radio applications a very robust detection scheme is of great advantage. This can be achieved by employing a differential detection scheme instead of coherent detection with a carrier recovery loop. Frequency instabilities of RF oscillators which are operating at frequencies from 7 GHz up to 55 GHz can force a classical carrier loop out of lock resulting in an interruption of the transmission. In case of mobile radio it is quite difficult to track the carrier phase due to the rapidly changing channel, especially when TDMA is used. In this paper we present a new differential detection scheme for continuous phase modulation which shows a performance degradation in the range of only 1 dB compared to a coherent receiver. This is accomplished by using a modified whitened matched filter adapted to the nonlinear nature of the modulation scheme. Further improvements are achieved by exploiting correlations in the noise process and by using reduced-state sequence estimation to cancel intersymbol interference. Due to the state reduction technique the complexity of the baseband processor is dramatically reduced and thus made suitable for integration into an ASIC. The performance of this new detection scheme is evaluated by an analysis of the symbol error probability and compared to computer simulations and hardware measurements.